Mobile communication systems have been developed to provide a user the ability to communicate while on the move. With the rapid advance of technologies, mobile communication systems have evolved such that they are now capable of providing a high speed data communication service as well as a voice telephony service.
FIG. 1 is a diagram illustrating a wireless communication system according to the related art.
Referring to FIG. 1, the wireless communication system includes a radio access network 130 and a core network 140. The radio access network 130 includes a base station 120. A terminal 100 and the base station 120 are connected through a radio link 110, and other nodes of the wireless communication system are connected through wired links. The base station 120 includes one or more cells, each cell has a certain service coverage area, and the terminal 100 is served within the cell coverage. Here, the cell means the cell of the cellular communication system and although the base station 120 is a device for managing and controlling the cell, the terms “base station” and “cell” are used interchangeably for convenience.
If the terminal 100 moves out of the range of the serving cell or if it is predicted that the terminal 100 will move out of the range of the serving cell, a new cell prepares to serve the terminal 100 to seamlessly provide the terminal with service. This process of changing the serving cell is referred to as handover. The cell serving the terminal 100 before the handover is called the source cell, and the cell to serve the terminal 100 after the handover is called the target cell.
The terminal 100 measures signals from the cells and reports the measurement results to the serving cell. The terminal 100 may measure signals of the serving cell and one or more neighbor cells. The cell receiving the measurement report makes a handover decision based on at least one of the reported measurement information and a previously stored mobility parameter. If the mobility parameter is set to an appropriate value, the handover is triggered at an appropriate time.
FIGS. 2A, 2B, and 2C are diagrams illustrating situations of connection failure due to the mobility parameter set inappropriately according to the related art. The connection failure may occur when the handover is not triggered at a supposed time (Radio Link Failure, RLF) or in the middle of the handover process (HandOver Failure, RLF).
FIG. 2A is a message flow diagram illustrating a situation of Too Late Handover (TLH) according to the related art.
In FIG. 2A, the mobility parameter of the cell 200 is configured so as to have a tendency of triggering handover too late. In this case, the cell 200 may continue serving the terminal 215 which is no longer in the service range of the cell 200 so as to cause connection failure as denoted by reference number 210. The terminal 215 may establish a connection to another cell 205 after the connection failure 210. Since the TLH has occurred due to the misconfigured mobility parameter of the cell 200, it is necessary to adjust the mobility parameter of the cell 200. In FIG. 2A, the situation is expressed in such a way that TLH has occurred to the cell 205.
FIG. 2B is a message flow diagram illustrating a situation of Too Early Handover (TEH) according to the related art.
In FIG. 2B, the mobility parameter of the cell 220 is configured so as to have a tendency of triggering handover too early. In this case, the cell 220 may forcibly trigger a handover of the terminal 240 which is still in the service range of the cell 220. This may cause connection failure after successful handover as denoted by reference number 230 or connection failure in the middle of handover process as denoted by reference number 235. The terminal 240 re-establishes a connection with the cell 220 after experiencing one of the connection failures after the successful handover and in the middle of the handover process. Since the TEH has occurred due to the misconfigured mobility parameter of the cell 220, it is necessary to adjust the mobility parameter. In FIG. 2B, the situation is expressed in such a way that TEH has occurred to the cell 225.
FIG. 2C is a message flow diagram illustrating a situation of Handover to Wrong Cell (HWC) according to the related art.
Referring to FIG. 2C, if the mobility parameter of the cell 250 is configured so as to have a tendency of triggering handover to a cell 255 configured with an incorrect mobility parameter, the cell 250 may trigger the handover to the cell 255, which is the wrong cell, rather than the cell 270, whose service coverage area the terminal 275 has actually entered, so as to cause connection failure after successful handover as denoted by reference number 260 or connection failure in the middle of handover process as denoted by reference number 265. The terminal 275 establishes a connection to the cell 270 after experiencing one of the connection failures after successful handover and in the middle of handover process. Since HWC has occurred due to the misconfigured mobility parameter of the cell 250, it is necessary to adjust the mobility parameter. In FIG. 2C, the situation is expressed in such a way that HWC has occurred to the cell 255 or HWC has occurred instead of handover to the cell 270.
It is possible to reduce the frequency of the occurrence of TLH, TEH, and HWC by transmitting, by the cell which has failed the attempted connection, the connection failure information to a new target cell, delivering the connection failure information to the cell whose mobility parameter is necessary to be adjusted, and adjusting the mobility parameter of the corresponding cell. This means the improvement of mobility robustness. Long Term Evolution (LTE) defines a connection failure detection mechanism of the cell of which mobility parameter adjustment is required only when the User Equipment (UE) which has failed connection attempts Radio Resource Control (RRC) re-establishment to a new cell.
However, the current handover mechanism has drawbacks in that 1) the connection failure information stored in the UE may be transmitted, with partial omission, to the cell requiring mobility parameter adjustment, 2) the terminal cannot detect any connection failure without attempt of RRC re-establishment, and 3) a method of adjusting the mobility parameter after detecting the connection failure is not specified. Therefore, there is a need for an apparatus and a method capable of improving the mobility robustness to overcome the above problem.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.